Electromagnetic friction device



United States Patent ELECTROMAGNETIC FRICTION DEVICE Paul A. Harter,Beloit, Wis., assignor to Warner-Electric Brake & Clutch Company, SouthBeloit, lll., a corporation of Illinois Application March 1, 1954,serial No. 413,153

z claims. (ci. 192s4) This invention relates to electromagnetic frictiondevices of the type having relatively rotatable magnet and armatureelements which are adapted for axial gripping engagement uponenergization of a winding of the magnet and which, upon deenergizationof the winding, are separated by a spring action to avoid rubbingcontact when the device is not producing a driving or retarding torque.More particularly, the invention is directed to a friction device andcontrol of the type shown in Winther Patent, No. 2,395,772, in which acapacitor is utilized to provide a momentary surge of current in thewinding upon closure of an energizing circuit for the latter to overcomethe gap between the friction faces and draw the magnetic elements intogripping engagement without sacricing subsequent control over thefriction coupling between the elements.

The general object of the invention is to provide an improved frictiondevice and control of the above character which enables a substantiallysmaller and less expensive capacitor to be used under all conditionsarising4 in service use.

Another object is to provide a novel control circuit for a device of theabove character in which the voltage source for the magnet winding isutilized more eiectually in deriving the energy for producing thecurrent surge and in which such energy is accumulated while the magnetwinding is deenergized so as to be available immediately upon closure ofthe energizing circuit for the winding.

The invention also resides in the novel manner of correlating thecharacteristics of the control and the friction device so that theaction of the capacitor is not accted by wear of the friction facesoccurring during service use.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in which Figure l is a diametrical sectional viewand a wiring diagram of a friction device and control therefor embodyingthe novel features of the present invention.

Fig. 2 is an enlarged fragmentary view similar to Fig. 1 showing theparts in different positions.

Fig. 3 is a schematic wiring diagram of a modified circuit arrangement.

The invention is shown in the drawings in conjunction with anelectromagnetic friction brake having relatively rotatable magnet andarmature members and 11 adapted to be drawn into axial grippingengagement upon energization of a multipie turn coil or winding 12. Themagnet 1i) comprises a rigid ring i3 of magnetic material of U-shapedcross section having radially spaced pole pieces 14 enclosing thewinding 12 and terminating in axially facing pole faces 15 which aredisposed in a common plane. Segments 16 of suitable frictionA materialsuch as brake lining are disposed between the pole pieces 14 and seatedagainst shoulders 17 with their outer faces flush with the pole faces15. The magnet 10 is stationarily mounted through the medium of a plate'18 secured rice to a stationary support 19 and fastened, as by welding,to the closed end of the magnet ring.

The armature 11 bridges the pole faces 15 to provide a substantinallyclosed toroidal flux circuit around the winding 12 and may comprise aseries of segments or a solid ring of magnetic material as shown securedto a disk 20 projecting inwardly from the armature and welded to acollar 21. To support the armature for rotation and axial slidingmovement relative to the magnet 10, bearing bushings 22 extendingaxially through apertures in the collar slidably receive a plurality ofangularly spaced pins 23 projecting rigidly from a support 24 which isfast on a rotatable shaft 25. Release springs 26 encircling the pinsyieldably urge the armature axially away from the magnet to a positionwhich is determined by a stop 27 and in which the friction faces of themagnet and armature are out of rubbing contact with each other and areseparated axially by a narrow air gap 28.

In the present instance, the gap 28 is maintained of a constant widthand the stop 27 comprises a plurality of split rings of resilientmaterial one contracted around and frictionally gripping each armaturesupporting pin 23 with a force greater than that exerted by the releasesprings 26 but less than the magnetic attractive force resulting fromenergization of the winding 12. The release springs act between heads29'on the pins and the outer surfaces of dished washers 30 encirclingthe pins with their outer oifset peripheral portions 31 bearing againstthe collar 21. On their inner peripheral portions, the washers provideaxially facing abutments 32 which, when the washers abut the collars 21,are spaced axially from the latter and opposed abutments 33 formed bythe inner ends of the bushings 22. One stop ring is disposed betweeneach pair of abutments 32 and 33 and its axial thickness is less thanthe spacing of the latter by an amount equal to the desired width of theair gap 28 which may be on the order of im of an inch. To hold thearmature ring 11 against drifting away from the stops 27 when the magnetis deenergized, compression springs 34 which are lighter than therelease springs 26 and are coiled about the bushings 22 bear at oppositeends against the armature support 24 and the disk 20. Herein, eachfollow-up spring 34 is adapted to exerta force of about 1A. pound ascompared to the 2 to 4 pounds force of each release spring 26.

Direct current for energizing the brake Winding 12 to produce ux in thetoroidal circuit extending around the U-shaped magnet ring and axiallyback and forth between the pole faces 15 and the armature 11 is suppliedby a source 35 which, in this instance, comprises a full wave rectifier36 whose input terminals are connected to a suitable alternating currentsource 37 and whose output terminals are connected to the windingthrough a series circuit including a switch 38 and a resistor 39. Thelatter is variable to control the voltage applied across the winding andthus, the value of current flowing in the latter. To maintain controlover the torque produced by a friction device of the above characterwhen the latter is energizedrinitially by closure of the switch 38, itis desirable to introduce energy to the winding 12 for producing amomentary surge of current therein suticient to overcome the air gap 28and the release spring action and draw the friction elements intogripping engagement and, then, to transfer the control to the variableresistor 39 which limits the current to a low value less than thatrequired to overcome-the gap but sucient to maintain'the grippingengagement of the friction elements. y K p In accordance with thepresent invention, the circuit elements controlling energization of themagnet winding 12 are arranged in a novel manner to enable thesamedirect current voltage source 35 which is utilized forV serviceoperation of the brake at low values of voltage and current to beutilized, while the winding is deenergized, for accumulating energy toproduce the required initial current surge. By storing such energyduring deenergization of the winding, the energy is available fordelivery at a high voltage equal approximately to the full voltage ofthe source and immediately upon completion of the low voltage circuit byclosure of the switch 38. At such a high voltage, a capacitor 40 ofsmall size and, therefore, low cost may be utilized to store the energyand still deliver the same in the proper amount and at the proper rateto overcome the air gap 28 and draw the armature 11 into grippingengagement with the pole faces 15.

To carry out the invention, the capacitor 40, in the circuit shown inFig. 1, is connected in parallel with the brake winding 12 and theswitch 38 and in series with the variable current controlling resistor39. By this arrangement, the capacitor is connected continuously in acharging circuit which includes the rectifier 36, the current varyingresistor 39, and the capacitor in series and through which the capacitorbecomes charged to substantially the full voltage of the source 36 whenthe switch 38 is open. The value of this voltage, in the presentinstance where a rectified source is used to provide direct current, isequal approximately to the peak value of the alternating current source37. This energy, stored in the capacitor when the switch 38 is open, isdischarged through another circuit including the capacitor, the winding,and the switch in series, the switch thus controlling both the lowvoltage and the capacitor discharge circuits.

In one electromagnetic friction brake and control of the above characterwhere the voltage rating of the brake is 90 volts, approximately 25 to30 volts must be applied to the winding to overcome the release springs26 and an air gap 2S of approximately ln of an inch between the frictionfaces. However, a voltage as low as 7 volts steady state is sufficientto hold the friction elements in engagement after the gap 28 has beenclosed. Where the voltage of the alternating current source 37 isapproximately 115 volts and the steady state unidirectional voltageoutput of the rectifier 36 is 90 volts, the capacitor 4t) will becomecharged to about 150 volts when the switch 38 is open as shown inFig. 1. At this voltage, a capacitor of 30 microfarads has been foundlarge enough to store and deliver suiiicient energy at the proper rate tdraw the friction elements into gripping engagement when the switch isclosed.

In the operation of the improved friction brake and control describedabove, let it be assumed that the switch 3S is open and winding 12 isenergized. Under this condition, the washers 30 are urged axially awayfrom the magnet by the release springs 26 and into abutment with thestop rings 27 .and the follow-up springs 34 hold the armature collar 2lagainst the outer peripheries'of the washers so that the armature faceis spaced the desired distance from the pole faces as shown in Fig. 2.The capacitor becomes charged through the resistor 39 to the fullvoltage of the source 35 and remains so charged as long as the switch isopen.

Now, when the switch 38 is closed, the stored energy in the capacitor 40is discharged through the winding 12 and the low voltage circuit for thelatter is completed. As a result of the energy discharge, the armature11 is shifted axially against the force of the release springs 26 andinto engagement with the pole faces 15 and the abutments 33 engage thestop rings 27 as shown in Fig. 1. Such energy discharge continues untilthe capacitor voltage reaches the value of voltage developed across thewinding through the low voltage circuit. The capacitor, being connectedin parallel with the Winding, remains charged at this lower voltageuntil the switch is opened.

As the friction faces of the magnet 10 and the armature 11 wear odduring engagement with each other, the stop lll rings 27 are shifted bythe abutments 33 axially along the pins 23 and toward the magnet adistance equal to the amount of the wear. The stops are retained in thenew positions through their frictional engagement with the pins so that,upon the next separation of the armature from the magnet, the air gap 28will be of the same axial width as before the wear occurred. Since theair gap is of uniform Width, the amount of energy required to draw thearmature 11 across the gap and, therefore, the size of capacitor 40needed to store this energy remain substantially the same throughout theservice life of the brake.

The invention can also be carried out where the winding 12 is energizedfrom a voltage divider circuit as shown in Fig. 3. Referring to thisfigure, the brake Winding 12 is connected in series with the switch 3Sbetween one output terminal of the rectifier 36 and the movable contactSii of a potentiometer 4Z having a resistance element 43 which isconnected in series with a second switch 41S and a current limitingresistor 45 across the rectifier output. As in Fig. l, the first switch33 is connected in series with the winding i2 across the capacitor 4d.The switches 38 and 44 are opened and closed in unison by a commonactuator 46.

in the modified circuit arrangement of Fig. 3, the capacitor di) will becharged to the voltage of the source 35 when the switches 3S and 44 areopen as shown in Fig. 3

through a series circuit including the capacitor, a part of thepotentiometer resistance element 43, and the current limiting resistor45. Upon ciosure of the switches, the discharge circuit of the capacitorthrough the winding is closed for discharge of the stored energy throughthe winding. To avoid dissipation of this energy through the secondswitch and the adjacent part of the potentiometer resistance d3, arectifier i7 poled to block the capacitor discharge current but to passcurrent for energizing the winding is interposed in series with themovable potentiometer contact 4i. between the latter and the conv monjunction 48 between the capacitor and the first switch 3S. Closure ofthe second switch 44 completes the circuit through the potentiometerresistance 43, the resistor 45, and the rectiiier 36 to apply to thewinding a voltage whose value is determined by the setting of thepotentiometer contact 41. The purpose of the second switch t4 is to openthe energizing circuit through the potentiometer 42 and enable thecapacitor to become charged to the maximum voltage of the source $5 atthe time that the first switch 3S is open to deenergize the winding.

By accumulating energy in the capacitor 4l) while the brake winding l2is deenergizcd so that the initial surge of energy may be delivered tothe winding at a voltage equal to the maximum value of the voltage ofthe direct current source 3S, the latter is utilized more effectuallythan in prior controls and the capacitor may be of substantially smallersize and still insure delivery of energy in the proper amount and at theproper rate to shift the armature 11 axially the length of the air gap28 and into engagement with the pole faces i5. By taking advantage ofthe wear compensating characteristic of the brake afforded by theshiftable stop 27, the capacitor 14 may bc of the same small size andinsure engagement of thc friction elements throughout the service lifeof the brake in spite of wearing od of the friction surfaces thereof.

, i claim as my invention;

l. The combination of, an electromagnetic friction device havingrelatively rotatable magnet and armature elements adapted for axialgripping engagement upon energization of a 'winding on the magnetelement and mounted for `axial sliding movement relative to and towardand away from each other, yieldable means urging said elements away fromeach other, a stop limiting axial separating movement of said elementsunder the action of said yieldable means, means operable during wearingoff of the friction faces of said elements .to shift said stop axiallyof the elements to compensate automatically for wear on the frictionfaces and maintain a gap of constant axial length between the faces upondeenergization of the winding, a circuit adapted when closed to energizesaid winding at a low voltage sucient to maintain said elements ingripping engagement but insuicient to overcome said yieldable means andsaid gap to draw the elements together to close the gap, a secondcircuit through said winding including a capacitor for storingelectrical energy for delivery at a voltage substantially higher thansaid low voltage and in an amount suicient to overcome said yieldablemeans and said gap, means for charging said capacitor with said energyfor delivery at said higher voltage while said circuits vare open, andmeans for closing said circuits substantially simultaneously todischarge said energy through said Winding to draw said elements intogripping engagement and then continue the energization of the winding atsaid low voltage to maintain such engagement.

2. The combination of, an electromagnetic friction device havingrelatively rotatable magnet and armature members adapted for axialgripping engagement upon energization of a winding on one of the membersand mounted for relative axial movement toward and away from each other,yieldable means urging said members apart axially, a stop limiting theaxial separating movement of said members under the action of saidyieldable means, means operating automatically to shift said stopaxially during wearing oi of the friction faces of the members tocompensate for such wear and maintain a gap of constant axial lengthbetween the faces upon deenergization of said winding, a circuit throughsaid Winding including a capacitor for storing electrical energy in antamount suicient, when the circuit is closed. to cause current to ow insaid winding at a predetermined value suicient to overcome said gap' andsaid yieldable means and draw said members into gripping engagement, asecond circuit adapted when closed to cause current `t0 ow in saidwinding `at a value less than said predetermined value but sufficient tohold said members in gripping engagement, means for charging saidcapacitor with said energy when said circuits are open, and means forclosing said circuits to discharge said energy of said capacitor throughsaid winding to draw said members into gripping engagement and thencontinue the energization of the winding through said second circuit tomaintain the members in such engagement.

References Cited in the tile of this patent UNITED STATES PATENTS2,290,406 Chambers July 21, 1942 2,395,772 Winther Feb. 26, 19462,427,751 Snyder Sept. 23, 1947 2,600,667 Mason June 17, 1952 2,664,782Weninger Jan. 5, 1954 2,675,507 Geiger Aug. 13, 1954 2,703,164 BinderMar. 1, 1955 2,718,951 Mason Sept. 27, 1955

